BRIDGE FORMULA DEVELOPMENT

The objective of this research was to review the existing bridge formula to determine whether modifications could be suggested to make it more rational. The intent was to more fully use the capacity of existing bridges without significantly shortening the service life of any. A formula, independent of the number of included axles, was developed to accomplish the objective. As is the current formula, it is applicable both to the overall wheelbase and to all included subgroups of axles. The maximum weights for single and tandem axles were assumed to be unchanged. If enforced, the proposed formula assures that HS 20 bridges will not be loaded to more than 1.05 times the design stress, nor will H 15 bridges be loaded to more than 1.30 times the design stress. The formula reduces the maximum weight allowed on four or more closely spaced axles. However, for most practical lengths, the formula is less restrictive than the current law. A brief study of the influence the proposed formula would have on pavement fatigue was accomplished. For most practical heavy vehicles, the formula would result in a greater number of equivalent axle loads per vehicle. One equivalent axle load causes the same pavement fatigue damage as a single 18,000-lb (80.06-kN) axle. The number of equivalent axle loads is commonly used as a measure of the fatigue damage a heavy vehicle inflicts on the pavement. A detailed study of the effect that the adoption of the proposed formula would have on pavements is recommended. Such a study should consider costs, benefits, and potential formula modifications.